4.1 Introduction to water systems

Earths Water Budget:

  • Blue planet (70% of Earth’s surface is water).
  • 97% is salt water and only 2.6% is fresh water.

Fresh water only makes up a small fraction (approximately 2.6% by volume) of the Earth's water storages.

The water cycle

Over 2/3rds of the earth’s surface is covered with water, but less than 1% is fresh water for drinking.
Fresh water is a scarce resource and is fast becoming a political issue in the developing world.

Flows in the hydrological cycle include evapotranspiration, sublimation, evaporation, condensation, advection (wind-blown movement), precipitation, melting, freezing, flooding, surface run-off, infiltration, percolation and stream-flow or currents

Human activities such as agriculture, deforestation, and urbanisation have a significant impact on surface run-off and infiltration

Human impact on the hydrological cycle:

  • Withdrawals: Domestic use, irrigation and industry
  • Discharges: Adding pollutants; sewage/fertilisers
  • Changing flow speed: Rivers are channelled underground; Canalising (straightening large sections); Dams/barrages/dykes
  • Diverting rivers: Away from important areas to avoid flood damage; Towards dams to improve storage

Human impact on water cycle

Ganges Basin

Aral sea:

Intense irrigation has almost stopped river flow into the sea. The sea level has lowered. The area of the Aral Sea has shrunk almost 90% in the last 50 years

Ocean circulation

Ocean circulation systems are driven by differences in temperature and salinity. The resulting difference in water density drives the ocean conveyor belt, which distributes heat around the world and thus affects climate.

  • Transports heat: equator to poles
  • Transport nutrients and organisms
  • Influences weather and climate
  • Influences commerce

Ocean Currents (horizontally and vertically)

Surface Currents: The upper 400 meters of the ocean (10%). Are moved by winds

Deep Water Currents: Thermal/Salinity currents (90%). When warm water rises, cold water goes down to replace it, and when cold water rises, warm water goes down to replace it).

Transport by currents:

Surface currents play significant roles in transport heat energy from equatorial waters towards the poles
Currents also involved with gas exchanges, especially O2 and CO2
Nutrient exchanges important within surface waters (including outflow from continents) and deeper waters (upwelling and downwelling)
Pollution dispersal
Impact on fisheries and other resources

Ocean currents and climate:

  • Water has a higher specific heat capacity than land.
  • Therefore takes longer to heat up/cool down.
  • This means land close to the oceans has a mild climate.
    E.g. The warm gulf stream/north Atlantic drift gives Britain (and NW Europe) a moderate climate when we should have a subarctic climate.

Great Ocean Conveyor Belt

The Great Ocean conveyor Belt is the name for a model of the large system of ocean currents that affects weather and climate by circulating thermal energy around Earth.
In this model, high salinity water cools and sinks in the North Atlantic, and deep water returns to the surface in the Indian and Pacific Oceans through upwelling
Scientists estimate that the Great Ocean Conveyor Belt model takes about 1,000 years

El Niño-Southern Oscillation (ENSO)

El Niño = warm surface current in equatorial eastern Pacific that occurs periodically around December
Southern Oscillation = change in atmospheric pressure over Pacific Ocean accompanying El Niño
ENSO describes a combined oceanic-atmospheric disturbance

La Nina

opposite El Nino

swings the other way and winds blow more than normal towards west

normal situation over Pacific Ocean (non-El Nino year):
low pressure over Asia
high pressure over South America
winds blow east to west most times of the year.
shallow thermocline along South (America, lots of upwelling, cold water at surface/Asia, warm surface water piles up)

Effects:

  • warm water with few nutrients rises
  • fisheries decline
  • increased precipitation over parts of South America, western and southern U.S.
  • can result in severe flooding
  • drought in Australia, Indonesia, Phillipenes
  • reduction in annual monsoons in India
  • can result in severe crop damage
  • severe property damage and thousands of deaths